1. Field of the Invention
The present invention relates to a communication system and a method for transmission and reception of a plurality of data different in quality of communication from each other including real-time data and nonreal-time data.
2. Description of Related Art
The ATM (Asynchronous Transfer Mode) communication system can provide an asynchronous communication, transmit data at a transfer rate covering a wide range from several tens Megabits per second (Mbps) to Gigabits per second (Gbps), and meets the requirements for a next-generation LAN (Local Area Network).
FIG. 1 shows an ATM terminal adopting an ATM communication of this type. As shown, the ATM terminal comprises a CPU (Central Processing Unit) 101, a memory 102 in which data to be transmitted are stored, a dual-port RAM 104, circuits 105, 108 and 111 to generate cells, circuits 106, 109 and 112 to add a header to each cell, traffic shaver circuits 107, 110 and 113, and an arbiter circuit 114.
The CPU 101 reads data from the memory 102 via a CPU bus 103, and transfers it to the dual-port RAM 104 which will supply three data to the cell generation circuits 105, 108 and 111, respectively, which will generate cells based on a size of PDU (Protocol Data Unit). The cells are supplied to the head add circuits 106, 109 and 112 which will add headers to the supplied cells, respectively, to generate ATM cells for delivery to the traffic shaver circuits 107, 110 and 113, respectively. In the traffic shaver circuits 107, 110 and 113, the supplied cells are subjected to a traffic shaving to adjust the ATM transfer rate to supply the cells to the arbiter circuit 114. The arbiter circuit 114 supplies the cells from the traffic shaver circuits 107, 110 and 113 to physical layers (not illustrated), respectively while re-arranging, by buffering, cells supplied at a same timing and also supplying them to the physical layers. More specifically, the arbiter circuit 114 arbitrates all connections in a same manner without discrimination of the quality of transmitted data, for example, whether they are real-time data or nonreal-time data.
The above ATM terminal is featured by maintaining a real-time occurrence of a data communication, namely, a necessary bandwidth for the communication. However, when supplied with ATM cells from the traffic shaver circuits 107, 110 and 113, respectively, at a same timing, the arbiter circuit 114 has to provide an arbitration to buffer one of the ATM cells. Therefore, jitters take place in the ATM cells. As shown in FIG. 2, for example, when real-time ATM cells are supplied more early than nonreal-time ATM cells, the arbiter circuit 114 will output the real-time ATM cells more early. On the other hand, when nonreal-time ATM cells are continuously supplied more early than real-time ATM cells, the arbiter circuit 114 will output the real-time ATM cells after outputting the nonreal-time ATM cells. In this case, however, the longer the continuation of the nonreal-time ATM cells, larger cumulated jitters will take place in the real-time ATM cells, which will impair the real-time occurrence of a data communication.
The present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing a communication system and a method for transmission of real-time and nonreal-time cells without any jitters caused in the real-time cells.
The above object can be accomplished by providing a communication system for outputting a first data to be real-time transmitted and a second data to be nonreal-time transmitted, comprising, according to the present invention, means for generating synchronous cells to output the first data synchronously with the communication system, means for generating asynchronous cells to output the second data, means for selecting either the synchronous or asynchronous cells, and means for controlling the selecting means to preferentially output either the synchronous or asynchronous cells.
According to another aspect of the present invention as set forth in claim 2, the controlling means controls the selecting means to preferentially output the synchronous cells.
Therefore, when synchronous cells are generated, the communication system according to the present invention will preferentially output the synchronous cells rather than asynchronous cells.